1. Field of the Invention
The present invention relates to halftone image recording methods and apparatus, and more particularly, to a halftone image recording method and apparatus improved in tone reproducibility.
2. Description of the Prior Art
In a halftone image recording apparatus controlled by an electronic circuit such as a process scanner provided with a dot generator, it is generally required to make screen pitches selectable according to the application of images to be recorded. In order to satisfy such a requirement, most of the halftone image recording apparatuses are constructed so that desired screen pitch can be selected.
However, when the scanning line pitch for recording is increased while correspondingly fixing the spot diameter of a recording laser beam at a relatively large size on a photosensitive material, output pixels arrayable in one halftone dot are decreased in number in case of a small screen pitch (hereinafter referred to as a "fine screen"), whereby required density expression grades cannot be obtained. When, to the contrary, the scanning line pitch is decreased while fixing the spot diameter of the beam at a relatively small size, the recording time is increased in the case of a large screen pitch (hereinafter referred to as a "coarse screen").
In order to solve such a problem, a technique is proposed in Japanese Patent Application No. 231109/1983, for example, in which the beam spot diameter is switched responsively to the screen pitch in a flat-bed type scanner by employing a beam expander. Further, disclosed in Japanese Patent Application No. 025155/1984 is such a technique whereby switching of the beam spot diameter in a drum type process scanner is provided by employing a zoom lens, etc.
However, it is not practical in the flat-bed type process scanner to prepare beams with various spot diameters corresponding to respective screen pitches when the apparatus is designed with various (e.g., six) selectable screen pitches. In the flat-bed scanning system, further, it is necessary to increase the focal length of an image forming lens of the optical system in order to obtain enough scanning length, and hence it is difficult to reduce the beam spot diameter to less than several tens of .mu.ms.
Therefore, it is necessary to select the relation between the recording beam spot diameter and the screen pitch so that halftone images for process printing can be recorded in a relatively short time using a relatively large recording beam spot diameter (i.e., recording scanning line pitch) without reducing density expression grades. Thus, in a process scanner of the flat-bed scanning type previously designed by the inventor herein, for example, two types of beam spot diameters of 34 .mu.m and 17 .mu.m are prepared and respectively used for the fine screen and the coarse screen.
However, there has been no technique for further increasing the density expression grades while maintaining the recording speed in both of the process scanner of the flat-bed scanning type and the drum-type, and no ideas have heretofore been proposed to solve such a problem.